The present invention relates generally to motor speed control systems and more particularly to such systems having high levels of response capability and control accuracy.
There have been proposed and available in the art various motor speed control systems wherein a prescribed or commanded speed of the motor is compared with a detected actual speed thereof and the power supply to the motor is controlled such that a difference between the actual and commanded speeds becomes zero.
Such motor speed control systems are characterized in that the power to be supplied to the motor is regulated, in response to variations in load applied to the motor, by means of feeding back the detected actual speed of rotation of the motor so as to maintain or stabilize the motor at a commanded nominal or desired speed. However, those systems in the art suffer potential problems or disadvantages as described below.
When such control systems consists of an analog circuit, for example, the individual systems require different adjustments appropriate for compensating for their variations in motor torque load, motor speed detector, specific circuit constants, etc. Another problem of the prior analog systems for speed control applications is that the controlled speed of the motor is affected by ambient conditions such as temperature and humidity, and subject to drifts of the circuit elements during service.
On the other hand, when the speed control system consists of a digital circuit, it is free from the above problem, but the system is limited in speed detecting time (sampling time) determined by a resolution of a detector used to detect the speed of rotation of the motor, and also limited in time necessary for processing associated digital signals. Therefore, such digital control system has a considerable difficulty in the speed control at a higher-than-audio frequency even when the controlling cycle time including the above times is reduced to its shortest possible length, whereby the prior digital control system is considered to have lower response capability than the analog control system.